In the case of network-controlled converters, as described, for example, in the applicant's AT 404 414 B, disturbances in the voltage or current, in particular overvoltages and/or commutation faults, can result in the destruction of the costly thyristors or switches of such a bridge.
In more precise terms, network-controlled converters are subject to the fundamental problem of inverter tripping in regenerative operation. Here, in the case of mains voltage drop and sufficient DC voltage, e.g. in the case of corresponding armature voltage of the motor, an overcurrent occurs, which increases still further, and can no longer be quenched by the converter itself. What happens then is the enforced actuation of the fuses which are provided to protect the thyristors. The consequence is lengthy downtime affecting the converter and the motor it supplies, as it takes a correspondingly long time to replace the fuses, which are customarily embodied as high-speed semiconductor fuses. Accordingly, efforts were made to create different, in some cases complex, devices to limit and interrupt the overcurrent or either to prevent inverter tripping, or terminate it in a controlled manner, as for example described in the said patent of the applicant.
DC current high-speed switches that lie in the DC current path are known, for example. If their secure functioning is to be guaranteed, however, highly controllable chokes are additionally required to limit the current rise, and the costs are correspondingly high, quite apart from the problem of dimensioning and the regular maintenance necessary for secure functioning.
Other methods for direct thyristor quenching with quenching condensers are known, for example the quenching of just one of the two halves of the converter bridge with one condenser, in which the motor current continues to exert a full load on the valves of the second bridge half, until complete reduction to zero, so that protective is not possible in all cases. A similar method is the quenching of both bridge halves with two condensers, and with a voltage limiter in each case parallel to these, although this allows overvoltages to occur in the motor. Furthermore, the named methods do not protect against overvoltage in the case of a mains outage if a transformer lies between the network and the converter bridge.